Method for estimating a time invariant transmission channel, and corresponding receiver

ABSTRACT

A method is for processing an analog channel signal from a transmission channel. The method may include converting of the analog channel signal to a digital channel signal, and performing a channel estimation digital processing of the digital channel signal. The channel estimation digital processing may include for at least one frame, generating transfer functions of the transmission channel, the transfer functions respectively associated with reference symbols of the frame, and averaging processing of the transfer functions to generate an average transfer function. The method may include decoding of symbols of the frame following the reference symbols using the average transfer function.

RELATED APPLICATION

This application is based upon prior filed copending French ApplicationNo. 1556489 filed Jul. 9, 2015, the entire subject matter of which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a method for transmission ofinformation over a transmission channel, and in particular, totransmission over a power line and related devices.

BACKGROUND

Power line communications (PLC) technology is aimed at transmittingdigital data by utilizing the existing infrastructure of the electricalgrid. It allows, in particular, remote reading of electric meters,exchanges between electric vehicles and the recharging terminals andalso management and control of energy networks (smart grid). PLCtechnology incorporates, in particular, narrow band power linecommunication (N-PLC) which is generally defined as a communication overan electrical line operating at transmission frequencies of up to 500KHz. N-PLC communication thus generally uses the frequency bands definedin particular by the European committee for electrotechnicalstandardization (CENELEC) or by the Federal Communications Commission(FCC). Thus, if the CENELEC A frequency band (3-95 kHz) is considered,the transmission frequencies are situated between 35.9375 and 90.625 KHzfor the PLC-G3 standard.

The signals conveyed by PLC and received by the receiver result from acombination of several signals having followed within the transmissionchannel (the electrical line) several propagation routes or paths eachhaving its own time delay and its own attenuation (the transmissionchannel is a multi-path transmission channel). The overall performanceof a receiver depends greatly on the quality of its channel estimation,i.e. on the estimation of the transfer function of this channel.Contemporary receivers, compatible with the PLC-G3 standard, aresuitable for performing a channel estimation when the latter is linearand time invariant (LTI: “Linear Time Invariant”), by using twoorthogonal frequency-division multiplexing (OFDM) symbols to estimatethe channel transfer function.

SUMMARY

Generally speaking, a method is for processing an analog channel signalfrom a transmission channel. The method may include converting of theanalog channel signal to a digital channel signal, and performing achannel estimation digital processing of the digital channel signal. Thechannel estimation digital processing may include for at least oneframe, generating a plurality of transfer functions of the transmissionchannel, the plurality of transfer functions respectively associatedwith a plurality of reference symbols of the at least one frame, andaveraging processing of the plurality of transfer functions to generatean average transfer function. The method may include decoding of symbolsof the at least one frame following the plurality of reference symbolsusing the average transfer function.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 7 are schematic diagrams of devices, according to the presentdisclosure.

FIGS. 8-11 are flowcharts illustrating operations of the devices,according to the present disclosure.

DETAILED DESCRIPTION

According to an embodiment, it is proposed to increase by several dB thedecoding performance of a receiver compatible, in particular with thePLC-G3 standard and connected to a channel of the LTI type. According toone aspect, a method is for processing an analog channel signal derivingfrom a transmission channel (e.g., an electrical line, the signal thenbeing conveyed by PLC). The analog channel signal conveys symbol frames,and the transmission channel is considered to be LTI for the entireduration of at least one frame. The method comprises an analog-digitalconversion of the analog channel signal so as to obtain a digitalchannel signal, a channel estimation digital processing, and a decodingof symbols performed on the digital channel signal. The channelestimation digital processing comprises, for the at least one frame, aformulation of at least three transfer functions of the channel, whichare respectively associated with at least three reference symbols of theat least one frame, and a processing for averaging the transferfunctions to obtain an average transfer function. The decoding of thesymbols of the at least one frame following the reference symbols isthen performed using this average transfer function.

The number of symbols on the basis of which the channel is estimated isincreased and an average of the transfer functions associated with thesesymbols is performed so as to use this average transfer functionthereafter in the decoding of the symbols. This makes it possible toincrease the signal-to-noise ratio of the average transfer function,which ultimately will lead to an increase in the decoding performance byseveral dB.

Even if the use of three reference symbols affords an improvement withrespect to the specifications of the PLC-G3 standard, it may be helpfulin practice to use a larger number of reference symbols to furtherimprove the decoding performance. It may be also advantageous to usesuccessive reference symbols.

In one embodiment, the channel estimation digital processing comprises,for the at least one frame, a formulation of M transfer functionsrespectively associated with M successive reference symbols of the atleast one frame, M being greater than or equal to 3, and preferablyconsiderably greater than 3, for example, greater than 10. The analogchannel signal which will undergo the analog-digital conversion may be,for example, the analog signal directly deriving from the channel orelse, as is generally the case, the analog signal delivered by an analoginput stage (in particular comprising band pass filters, low-passfilters and an amplifier) connected to the transmission channel. Theanalog channel signal is, for example, compliant with the PLC-G3standard.

In this case, the number M of reference symbols can be equal to 15 andthen together extend over a duration equal to 10.42 ms (since eachreference symbol has a duration equal to 0.695 ms). This total durationof 10.42 ms is slightly greater than half the period of the alternatingcurrent intended to flow on the electrical line (i.e. 20 ms for afrequency of 50 Hz). In the same manner, in countries where the mainsfrequency is 60 Hz, half the period of the carrier current is equal to8.33 ms and the number M of reference symbols can be taken equal to 12.The channel is considered to be time invariant for the entire durationof the at least one frame.

Of course, the channel may be time invariant in the course of severalframes, successive or not, and in this case, the various above-mentionedsteps of the method are advantageously applied for each of these frames,the channel being considered, during the reception of each of theseframes, to be time invariant for the entire duration of the frameconsidered. On the send side, the analog channel signal derives from adigital-analog conversion of an initial digital signal, and when thesampling frequency of the digital channel signal (on reception) isdifferent from the sampling frequency of the initial digital signal (onemission), it is preferable to take account of this shift of samplingfrequency (or “sampling frequency offset”) to correct the transferfunctions.

Thus, in one embodiment, the channel estimation digital processingcomprises a formulation of M base transfer functions respectivelyassociated with the M reference symbols of the frame received, and aprocessing for correcting these M base transfer functions with a phaseshift corresponding to this shift of sampling frequency so as to obtainthe M transfer functions. The received-frame reference symbols on thebasis of which the various transfer functions will be determined are,for example, symbols of the received frame corresponding to knownsymbols of the transmitted frame and/or are decodable symbols that canbe decoded without knowing the transfer function of the transmissionchannel.

Thus, in the case of the PLC-G3 standard, each received frame comprisesa preamble followed by a header followed by a useful field. Thereference symbols can comprise the symbols of the header of a frame,which are decodable without knowing the transfer function of thetransmission channel since they are coded in a differential manner, aswell as the two symbols of the useful field (“payload”) of the framewhich correspond to the two known symbols S1, S2 of the useful field ofthe transmitted frame.

It will therefore be noted here that although the PLC-G3 standardprovides for only two known symbols, namely the symbols S1 and S2, aspilot symbols for the estimation of the transfer function of thechannel, provision is advantageously made here to increase this numberof pilot symbols by using the symbols of the header of a frame which aredecodable without making it necessary to know the transfer function ofthe channel. It is then readily possible to refine the channelestimation and obtain several transfer functions which will be averagedthereafter. This being so, it would also have been possible to also useas reference symbols all or some of the known symbols of the preamble ofa frame so as to further increase the value of M and thus improve thechannel estimation. Moreover, each formulation of a transfer functionassociated with a decodable symbol received comprises a decoding of thesymbol received, a re-encoding of this decoded symbol so as to obtain are-encoded symbol, and a determination of the transfer function of thechannel on the basis of the re-encoded symbol and of the decodablesymbol received.

According to another aspect, a receiver may include an input stageintended to be connected to a transmission channel and configured todeliver an analog channel signal deriving from the transmission channel.The analog channel signal is intended to convey symbol frames, and thetransmission channel is considered to be LTI for the entire duration ofat least one frame. The receiver may include an analog-digitalconversion stage for converting the analog channel signal so as todeliver a digital channel signal, and a stage for processing the digitalchannel signal comprising channel estimation means or a channelestimator and means for decoding symbols or a decoder.

The estimation means may comprise formulation means or a formulatorconfigured to formulate, for the at least one frame, at least threetransfer functions of the channel which are respectively associated withat least three reference symbols of the at least one frame, and anaverage calculator block configured to perform a processing foraveraging the transfer functions obtained so as to obtain an averagetransfer function. The decoding means may be then configured to decodethe symbols of the at least one frame following the reference symbols byusing this average transfer function.

According to one embodiment, the formulation means may be configured toformulate, for the at least one frame, M transfer functions respectivelyassociated with M successive reference symbols of the at least oneframe, M being greater than or equal to 3. The analog channel signal mayderive from a digital-analog conversion of an initial digital signal,and when the sampling frequency of the digital channel signal isdifferent from the sampling frequency of the initial digital signal. Theformulation means may be further configured to formulate M base transferfunctions respectively associated with the M reference symbols of theframe received, and to correct these M base transfer functions with aphase shift corresponding to this shift of sampling frequency so as toobtain the M transfer functions. The reference symbols of the receivedframe can correspond to known symbols of the transmitted frame and/or bedecodable symbols that can be decoded without knowing the transferfunction of the transmission channel.

Additionally, the formulation means may be configured to formulate atransfer function associated with a decodable symbol received by adecoding of the symbol received, a re-encoding of this decoded symbol soas to obtain a re-encoded symbol and a determination of the transferfunction of the channel on the basis of the re-encoded symbol and of thedecodable symbol received. The signal can be modulated according to anOFDM modulation.

The transmission channel may be an electrical line, and the analogchannel signal is intended to be conveyed by PLC. The analog channelsignal can be compliant with the PLC-G3 standard. In this case, eachreceived frame comprises a preamble followed by a header followed by auseful field, and the reference symbols can comprise the symbols of theheader and two symbols of the useful field corresponding to two knownsymbols of the transmitted frame.

The modes of implementation and embodiments which will now be describedare described within the context of a transmission of information by PLCcompliant with the PLC-G3 standard, although the present disclosure isnot limited to this type of application. Throughout what follows, eachtime that the PLC-G3 standard is cited, it will be assumed that theCENELEC A frequency band (3-95 kHz) is considered.

Referring initially to FIG. 1, an exemplary sender capable oftransmitting a useful signal SU on an electrical line LE by PLC is nowdescribed. The transmission chain comprises, for example, an encoderENC, such as a convolutional encoder, receiving the data to betransmitted from source coding means or a source code store.Interleaving means or an interleaver INTL are connected to the output ofthe encoder and are followed by “mapping” means or a mapper whichtransform the bits into symbols according to a transformation schemedependent on the type of modulation used, for example, a modulation ofbinary phase-shift keying (BPSK) type or more generally a quadratureamplitude modulation (QAM) modulation. Each symbol contains modulationcoefficients associated with carriers which will be modulatedaccordingly. The symbols are delivered as input to processing means or aprocessor MTFI intended to perform an inverse fast Fourier transform(IFFT) operation.

It will be noted here, referring more particularly to FIG. 2, that themodulated carriers form a subset SNS of carriers from among an availableset ENS of carriers (which set corresponds to the size of the inverseFourier transform). Thus, in the PLC-G3 standard, the size of theinverse Fourier transform is equal to 256 while the modulated carriersof the subset SNS lie between the ranks 23 and 58, this corresponding toa frequency band F1-F2 lying between 35.9375 and 90.625 KHz. Thesampling frequency is here equal to 400 KHz leading to an inter-carrierspacing equal to 1.5625 KHz, this thereby rendering the frequenciesorthogonal (i.e. OFDM modulation). The modulation coefficientsassociated with the unused carriers are equal to 0.

The OFDM signal in the time domain is generated as output from theprocessing means MTFI, and means or circuitry MCP add, to each OFDMsymbol in the time domain, a cyclic prefix which is a copy at the headof the OFDM symbol of a certain number of samples situated at the end ofthis symbol. By way of example, in the PLC-G3 standard, the length ofthe cyclic prefix is 30 samples for a sampling frequency of 400 KHz. Thesignal is thereafter converted in a digital-analog converter CNA andthen processed in a stage ETA, commonly referred to by the personskilled in the art by the designation “Analog Front End”, where itundergoes in particular a power amplification, before being transmittedon the electrical line LE.

At reception, it is seen, by referring more particularly to FIG. 3, thathere the receiver RCP comprises an analog input stage ET1 whose inputterminal BE is connected to the electrical line LE. This analog inputstage ET1 comprises in a conventional manner a band pass filter BPF, alow-pass filter LPF, and amplification means or an amplifier AMP. Theoutput of the stage ET1 is connected to an analog-digital conversionstage CAN whose output is connected to the input of processing means orstage ET2.

The processing stage ET2 here comprises automatic gain control AGC meansor a circuit making it possible to control the value of the gain of theamplification means AMP of the stage ET1. The signal SAC delivered asoutput from the analog stage ET1 and as input to the analog-digitalconversion CAN stage designates an analog channel signal deriving fromthe transmission channel (electrical line) LE.

The processing stage ET2 also comprises a low-pass filter LPF2 followed,although this is not indispensable, by sub-sampling means or asub-sampler MSCH. The sampling frequency of the signal upstream of themeans MSCH is denoted as Fs while the sampling frequency of the signalat the output of the means MSCH is denoted as Fss.

The signal SNC output by the means MSCH then designates here a digitalchannel signal, which derives from the analog-digital conversion of theanalog channel signal SAC, and on which in particular a synchronizationprocessing, a channel estimation and decodings of symbols will beapplied as will be seen in greater detail hereinafter. The channelestimation is performed once synchronization has been acquired. Thefrequency Fc designates the calculation frequency at which the variousprocessing operations will be performed.

In the PLC-G3 standard, for example, the sampling frequency Fs specifiedis 400 KHz for an FFT size of 256. Although it would have been possibleto perform all the operations of these various processing operations ata calculation frequency Fc equal to the sampling frequency Fs of 400KHz, the fact of under-sampling the signal at a frequency Fss less thanFs and of performing all the operations at the calculation frequency Fcequal to Fss makes it possible to reduce the complexity ofimplementation of the processing stage and also makes it possible toperform a direct fast Fourier transform (FFT) processing having areduced size with respect to the specified size of 256.

Before returning in greater detail to the various means and circuitryincorporated into the processing stage ET2, now referring moreparticularly to FIG. 4, the structure of a frame conveying symbols, forexample, within the context of the PLC-G3 standard is now described. Theframe received TRM comprises a preamble PRM comprising here eight knownsymbols SYNCP followed by a symbol of opposite phase SYNCM, itselffollowed by a half-symbol SYNCM. The frame TRM thereafter comprises aheader HD followed by a useful field PLD containing symbols of usefuldata to be decoded and better known by the person skilled in the art asa “payload”. The symbols of the header HD contain, in particular,control information for the decoding of the data of the field PLD aswell as the number of bytes to be decoded in the field PLD.

The preamble PRM of the frame TRM allows the receiver to synchronizeitself, i.e. to obtain an indication IND1 making it possible to retrievethe structure of the frame so as to be able to tag the start of theheader HD. The transmission channel is a linear channel, i.e. that itbehaves as a linear filter. It is considered here furthermore that for aconsidered frame of the channel is LTI for the entire frame. Statedotherwise, the characteristics of its transfer function are invariantthroughout the frame.

In certain applications for which the electrical environment of thechannel is known (being, for example, devoid of objects connected to anelectrical line whose overall impedance is not predominated by one ormore objects whose electrical characteristics generate acyclo-stationary impedance, such as halogen lamps and/or voltagerectifiers), the transmission channel can be considered de facto to betime invariant in the course of the frames received. In otherapplications, the knowledge of the time invariant state of the channelin the course of the frame considered can result, for example, from aprior detection of the state of the channel, in particular but notlimited to that described in the French patent application in the nameof the Applicant, entitled “Procédé traitement d'un signal issu d'uncanal de transmission, en particulier un signal véhiculé par courantporteur en ligne, et notamment l'estimation du canal, et récepteurcorrespondant”, “Method for processing a signal deriving from atransmission channel, in particular a signal conveyed by PLC, and inparticular the estimation of the channel, and corresponding receiver”,and filed on the same day as the present patent application.

Now referring again to FIG. 3, it is seen that the processing stage ET2comprises a sub-stage ET20 incorporating various means and logiccircuitry that will now be described in a functional manner. Thesevarious means and logic circuitry can be achieved in a software mannerwithin a microprocessor for example, then forming at least in part thesub-stage ET20.

Featuring in a conventional manner, among these various means, aresynchronization means or a synchronizer MSYNC allowing the receiver tosynchronize itself, i.e. to obtain the indication IND1 making itpossible to retrieve the structure of the frame, so as to be able to tagthe start of the header HD. These synchronization means can be ofconventional structure known to the skilled person or else, as avariant, those incorporating the filtering means or a filter describedin French patent application No. 1552588.

Featuring among the other means incorporated into the sub-stage 20 arechannel estimation means NEST comprising formulation means MLBconfigured, as will be seen in greater detail hereinafter, to formulate,for the frame considered, several transfer functions of the channel thatare respectively associated with several reference symbols of the frame,and an average calculator block MMY configured to perform a processingfor averaging the transfer functions obtained so as to obtain an averagetransfer function. Decoding means MDCD of conventional structure knownto the skilled person are then configured to decode the symbols of theframe following the reference symbols by using this average transferfunction.

Referring now more particularly to FIG. 5, it is seen that the header HDof the frame received TRM comprises thirteen symbols FCH₁-FCH₁₃, whichhave been coded on emission in a differential manner and which are eachreferenced with respect to the preceding symbol. The frame TRM moreovercomprises, at the start of the useful field PLD, two symbolscorresponding to two known transmitted symbols S1, S2. That being so,for the sake of simplification these two symbols received will also bedesignated by S1 and S2. The thirteen symbols FCH_(i) and the twosymbols S1 and S2 here form M reference symbols SYMR_(i) (M is equal to15 in this example). These reference symbols will be used to formulate Mtransfer functions which will thereafter be averaged to obtain theaverage transfer function mentioned hereinabove.

In FIG. 6, the curve CV schematically represents the periodic variationsof the absolute value of the carrier signal flowing on the electricalline (alternating current or voltage) and, in this figure, PS/2designates half the period of this carrier signal. Thus, for a currentand voltage alternating at 50 Hz, PS/2 is equal to 10 ms. Since, in thePLC-G3 standard, each reference symbol SYMR_(i) has a duration equal to0.695 ms, all of the 15 reference symbols SYMR₁-SYMR₁₅ extend temporallyover a total duration D equal to 10.42 ms which in the particularexample described here is slightly greater than PS/2.

Stated otherwise, in the present case, half the period of the carriersignal is not an integer multiple of the duration of a reference symboland it lies between 14 times 0.695 ms and 15 times 0.695 ms. This beingso, the number M is not related to PS/2 and could optionally be greaterthan 15 in the case where other reference symbols could be used as, forexample, at least some of the symbols of the preamble of the frame.

Now, referring more particularly to FIGS. 7 to 11, the channelestimation phase is described in more detail. This estimation phase isperformed once the synchronization of the receiver has been acquired.This estimation phase is performed here at each frame reception forwhich the channel is time invariant, and now the processing operationsperformed in the course of one of these frames is now described.

As illustrated in FIG. 7, the channel estimation processing performed bythe estimation means NEST on the basis of the digital channel signal SNCwill make it possible to obtain an average channel transfer function HMwhich will be used for the decoding of the symbols following the lastreference symbol. In this regard, as illustrated in FIG. 8, theformulation means MLB formulate firstly in step 80 a transfer functionH_(i) of the channel for each reference symbol SYMR_(i).

More precisely, this transfer function H_(i) is equal to the product ofthe reference symbol received SYMR_(i) times the complex conjugate ofthe corresponding symbol transmitted by the sender on the transmissionchannel. In the present case, in compliance with the PLC-G3 standard,this transfer function H_(i) is in fact a complex vector having 36complex components corresponding respectively to the 36 tones of thesymbol.

On completion of step 80, an initial sequence of M (=15) transferfunctions H₁-H₁₅ is therefore obtained, associated respectively with theM (=15) reference symbols SYMR₁-SYMR₁₅. In the embodiment which has justbeen described, it was assumed that the send side sampling frequency wasidentical to the sampling frequency of the digital channel signal SNC.That being so, the sampling frequency of the digital signal SNC may bedifferent from the initial digital signal formulated in the sender.

This case results in a shift of sampling frequency known to the personskilled in the art by the expression “sampling frequency offset” thathas to be taken into account in the estimation of the transfer functionsH_(i). This is shown in FIG. 9. More precisely, base transfer functionsHB_(i) respectively associated with the reference symbols SYMR_(i) aredetermined in step 800 in a manner analogous to what was described instep 80 of FIG. 8.

Next, the frequency shift (“sampling frequency offset”) is estimatedusing, for example, two base transfer functions that are relatively farapart temporally, for example, the transfer functions HB₁ and HB₁₃. Theresulting phase shift is then obtained by performing the product of thetransfer function HB₁ times the complex conjugate of the transferfunction HB₁₃, the whole divided by the number of symbols.

A phase shift correction DPHC is then obtained, which is applied in astep 801 to correct the M transfer functions HB_(i) and to obtain the Mtransfer functions H_(i). The estimation of a transfer function on thebasis of each of the symbols S1 and S2 can be performed withoutdifficulty since the transmitted symbols corresponding to the symbols S1and S2 received are known.

On the other hand, such is not the case for the reference symbols of theheader, i.e. the symbols FCH_(i). Nonetheless, as indicated hereinabove,these symbols FCH_(i) were coded at the sending end in a differentialand particularly robust manner. Their decoding does not thereforerequire knowledge of the channel transfer function.

FIG. 10 illustrates an example of estimating a transfer function H_(i)of the channel on the basis of the symbol FCH_(i) of the headerreceived. The decoding (step 100) of the symbol received FCH_(i) isundertaken firstly. In this regard, the decoding means conventionallycomprise means configured to remove the cyclic prefix from each symbol,followed by means configured to perform the direct fast Fouriertransform FFT.

The decoding means also comprise demapping means or a demapper providingfor each carrier a value of the corresponding modulation coefficient(bin). These demapping means are followed by a module configured todetermine for each modulation coefficient an indication of confidence(soft decision) of the value. This module is conventional and known tothe skilled person and uses, for example, an algorithm of the LogMAPtype.

The decoding means also comprise deinterleaving means or a deinterleaverfollowed by a decoder, for example, a decoder of Viterbi type, followedby means or a circuit able to perform a parity check. The output ofthese means is connected to the sub-stage ET20 output terminal BS whichis connected to the means forming the MAC layer of the receiver.

Since the various symbols FCH_(i) are referenced with respect to thepreceding symbols in the frame, it is necessary to decode all thesymbols FCH_(i) of the header with the decoding means mentionedhereinabove. Next, after verification that the parity check is correct,it is possible to obtain the various decoded symbols FCHD_(i).

A re-encoding of each of these symbols FCHD_(i) is then performed instep 101 by using a convolutional encoder, an interleaver and a mappingmeans analogous to the corresponding means ENC, INTL, MP illustrated inFIG. 1 for the sender part. It will be noted in this regard that weremain in the frequency domain.

Re-encoded symbols FCHEC_(i) corresponding to the transmitted symbolsare then obtained. It is then possible, in step 80 analogous to step 80of FIG. 8, to obtain the transfer functions H_(i) associated with thevarious symbols FCH_(i) on the basis of these received symbols FCH_(i)and of the re-encoded symbols FCHEC_(i).

Instead of simply using, as mentioned in the PLC-G3 standard, thesymbols S1 and S2 of a frame to estimate the transfer function of thechannel, it is possible, as illustrated in FIG. 11, advantageously touse the M (M=15) transfer functions H_(i) associated with the 15reference symbols to calculate an average (step 150) thereof in theaverage calculator block MMY (FIG. 3) so as to obtain the averagetransfer function HM which will then be used for the decoding (step 151)of the symbols P0, P1, . . . of the useful field PLD of the frame. Thistransfer function HM is taken into account for the decoding, as is knownby the person skilled in the art, at the level of the demapping meansincorporated into the decoding means. The signal-to-noise ratio of thetransfer function HM is thus significantly increased, this ultimatelybeing manifested by an improvement of several dB in the decodingperformance.

1-18. (canceled)
 19. A method for processing an analog channel signalfrom a transmission channel, the analog channel signal conveying symbolframes, the transmission channel being linear and time invariant for aduration of at least one frame, the method comprising: analog-digitalconverting of the analog channel signal to generate a digital channelsignal; performing a channel estimation digital processing of thedigital channel signal; the channel estimation digital processingcomprising for the at least one frame, generating at least threetransfer functions of the transmission channel, the at least threetransfer functions respectively associated with at least three referencesymbols of the at least one frame, and averaging processing of the atleast three transfer functions to generate an average transfer function;and decoding of symbols of the at least one frame following the at leastthree reference symbols using the average transfer function.
 20. Themethod according to claim 19 wherein the channel estimation digitalprocessing comprises, for the at least one frame, generation of Mtransfer functions respectively associated with M successive referencesymbols of the at least one frame, M being greater than or equal to 3.21. The method according to claim 20 wherein the analog channel signalis based upon a digital-analog conversion of an initial digital signal;and wherein when a sampling frequency of the digital channel signal isdifferent from a sampling frequency of the initial digital signal, thechannel estimation digital processing comprising: generating M basetransfer functions respectively associated with the M successivereference symbols of the at least one frame; and correcting the M basetransfer functions with a phase shift corresponding to a shift ofsampling frequency so as to obtain the M transfer functions.
 22. Themethod according to claim 19 wherein the at least three referencesymbols of the at least one frame correspond to known symbols; andwherein the at least three reference symbols are decodable symbols to bedecoded without knowing a transfer function of the transmission channel.23. The method according to claim 22 wherein each generation of atransfer function associated with a decodable symbol received comprises:decoding the symbol received; re-encoding the decoded symbol to obtain are-encoded symbol; and determining the transfer function of thetransmission channel based upon the re-encoded symbol and the decodablesymbol received.
 24. The method according to claim 19 wherein the analogchannel signal is modulated according to an Orthogonalfrequency-division multiplexing (OFDM) modulation.
 25. The methodaccording to claim 19 wherein the transmission channel comprises anelectrical line; and wherein the analog channel signal is conveyed bypower line communications.
 26. The method according to claim 25 whereinthe analog channel signal complies with the power line communicationsPLC-G3 standard.
 27. The method according to claim 22 wherein eachreceived frame comprises a preamble, a header following the preamble, auseful field following the header; wherein the at least three referencesymbols are in the header; and wherein at least one symbol of the usefulfield corresponds to at least one known symbol of the at least oneframe.
 28. A method for processing an analog channel signal from atransmission channel, the method comprising: converting of the analogchannel signal to a digital channel signal; performing a channelestimation digital processing of the digital channel signal; the channelestimation digital processing comprising for at least one frame,generating a plurality of transfer functions of the transmissionchannel, the plurality of transfer functions respectively associatedwith a plurality of reference symbols of the at least one frame, andaveraging processing of the plurality of transfer functions to generatean average transfer function; decoding of symbols of the at least oneframe following the plurality of reference symbols using the averagetransfer function.
 29. The method according to claim 28 wherein thechannel estimation digital processing comprises, for the at least oneframe, generation of M transfer functions respectively associated with Msuccessive reference symbols of the at least one frame, M being greaterthan or equal to
 3. 30. The method according to claim 29 wherein theanalog channel signal is based upon a digital-analog conversion of aninitial digital signal; and wherein when a sampling frequency of thedigital channel signal is different from a sampling frequency of theinitial digital signal, the channel estimation digital processingcomprises: generating M base transfer functions respectively associatedwith the M successive reference symbols of the at least one frame; andcorrecting the M base transfer functions with a phase shiftcorresponding to a shift of sampling frequency so as to obtain the Mtransfer functions.
 31. A receiver comprising: an input stage to becoupled to a transmission channel and configured to deliver an analogchannel signal from the transmission channel, the analog channel signalconveying symbol frames; an analog-digital converter configured toconvert the analog channel signal to a digital channel signal; and aprocessor configured to perform a channel estimation digital processingof the digital channel signal, for at least one frame, generate aplurality of transfer functions of the transmission channel, theplurality of transfer functions respectively associated with a pluralityof reference symbols of the at least one frame, average the plurality oftransfer functions to generate an average transfer function, and decodeof symbols of the at least one frame following the plurality ofreference symbols using the average transfer function.
 32. The receiveraccording to claim 31 wherein the transmission channel is linear andtime invariant for a duration of the at least one frame.
 33. Thereceiver according to claim 31 wherein said processor is configured tofor the at least one frame, generate of M transfer functionsrespectively associated with M successive reference symbols of the atleast one frame, M being greater than or equal to
 3. 34. The receiveraccording to claim 33 wherein the analog channel signal is based upon adigital-analog conversion of an initial digital signal; and wherein saidprocessor is configured to: when a sampling frequency of the digitalchannel signal is different from a sampling frequency of the initialdigital signal, generate M base transfer functions respectivelyassociated with the M successive reference symbols of the at least oneframe, and correct the M base transfer functions with a phase shiftcorresponding to a shift of sampling frequency so as to obtain the Mtransfer functions.
 35. The receiver according to claim 31 wherein theplurality of reference symbols of the at least one frame correspond toknown symbols; and wherein the plurality of reference symbols aredecodable symbols to be decoded without knowing a transfer function ofthe transmission channel.
 36. The receiver according to claim 35 whereineach generation of a transfer function associated with a decodablesymbol received comprises: decoding the symbol received; re-encoding thedecoded symbol to obtain a re-encoded symbol; and determining thetransfer function of the transmission channel based upon the re-encodedsymbol and the decodable symbol received.
 37. The receiver according toclaim 31 wherein the analog channel signal is modulated according to anOrthogonal frequency-division multiplexing (OFDM) modulation.
 38. Thereceiver according to claim 31 wherein the transmission channelcomprises an electrical line; and wherein the analog channel signal isconveyed by power line communications.
 39. The receiver according toclaim 38 wherein the analog channel signal complies with the power linecommunications PLC-G3 standard.
 40. The receiver according to claim 35wherein each received frame comprises a preamble, a header following thepreamble, a useful field following the header; wherein the plurality ofreference symbols are in the header; and wherein at least one symbol ofthe useful field corresponds to at least one known symbol of the atleast one frame.